1. Field of the Invention
The present invention relates to a sheet processing device, an image forming system, and a method of additionally folding a sheet bundle, and specifically relates to a sheet processing device having a function for folding a sheet recording medium such as paper, recording paper, and transfer paper (hereinafter, simply referred to as a “sheet” in this specification), an image forming system including the sheet processing device, and a method of additionally folding a sheet bundle performed by the sheet processing device.
2. Description of the Related Art
In the related art, provided are postprocessing devices used in combination with image forming apparatuses such as a copying machine for binding a saddle-stitched booklet by folding one sheet, or by stitching the center part in the longitudinal direction of a sheet bundle including a plurality of sheets and folding the center part of the sheet bundle with a pair of folding rollers arranged in parallel to a sheet folding direction. An additional-folding technique is already known for reinforcing a fold line part of a saddle-stitched booklet after folding processing by an additional-folding roller moving along the fold line part after saddle-stitching and center-folding are performed.
As an example of such an additional-folding technique, known is a technique disclosed in Japanese Laid-open Patent Publication No. 2012-153530.
The technique provides a folding roller unit that forms a fold line on a sheet while passing through a nip of a pair of rollers, and an additional-folding roller unit including a first roller arranged on a first surface side orthogonal to a carrying direction of the sheet bundle folded by the folding roller unit and second and third rollers that are arranged on a second surface side different from the first surface orthogonal to the carrying direction of the folded sheet bundle and form a nip with the first roller. The technique also provides a driving unit that moves the additional-folding roller unit along the fold line part in a state in which the folded sheet bundle is held in the nip between the first roller and the second roller and the nip between the first roller and the third roller.
The additional-folding roller unit disclosed in Japanese Laid-open Patent Publication No. 2012-153530 includes, specifically, three additional-folding rollers to be driven along the fold line while holding the fold line part of the sheet bundle. In this case, a diameter of the first roller is larger than each of diameters of the second roller and the third roller. When three folding rollers are used as described above, two nips are formed, and tangential directions of the nips are not parallel to each other. Due to this, each of a line connecting the center of the large-diameter first roller and the center of the second roller and a line connecting the center of the large-diameter first roller and the center of the third roller is shifted from a thickness direction of sheets. The fold line part is reinforced due to the shift.
In this case, it is considered that good additional-folding strength can be obtained by changing an angle of the shift corresponding to the thickness of the sheet bundle or presence or absence of a staple. However, in the technique disclosed in Japanese Laid-open Patent Publication No. 2012-153530, additional-folding is basically performed at substantially a constant angle although an angle of the tangential direction of the nip is changed in some degree depending on the sheet thickness. This has caused deformation of the staple or drooping of an end of the sheet bundle. The deformation of the staple or the drooping has impaired stacking property of the sheet bundle.
In view of the above-mentioned conventional problems, there is a need to prevent the deformation of the staple or the drooping of the end of the sheet bundle in additional-folding, and prevent the stacking property of the sheet bundle from being impaired.